In her 2014 book, Generation Unbound: Drifting Into Sex and Parenthood Without Marriage, Isabel V. Sawhill argues that unplanned births are a main cause of poverty, and that one of the most effective ways to reduce poverty (as well as inequality) is to help women, particularly young women, prevent unplanned pregnancies.

This is hardly a new idea, but Sawhill's research has given it more heft, and anti-poverty funders should be paying close attention. While reducing unplanned pregnancies isn't easy, it's arguably a much lighter lift than tackling many of the other factors that underlie poverty, and that's especially true in light of advances in contraception, as we'll see in a moment. Enabling women to better control their fertility is also a classic upstream intervention that forestalls the need to address a range of other social problems, delivering lots of bang for the buck. Still, for various reasons, many funders that work on poverty steer well away from this area.

The good news, though, is that one of the biggest foundations in America, the Susan Thompson Buffett Foundation (STBF), is squarely focused on sexual and reproductive health and rights. Flying well beneath the radar, without any real web presence, STBF is a lot bigger than most people realize, giving away $450 million in 2013. Only Gates and Ford gave away more that year, and our guess is that STBF's numbers will be even higher for 2014 and 2015, since it's powered by gifts of Berkshire Hathaway stock from Warren Buffett, and the value of such shares has shot up in recent years.

Which is one reason why we named Susie Buffett, the chair of the foundation (which is named after her mother), as the second-most powerful woman in U.S. philanthropy last year.

The Susan Thompson Buffett Foundation works both globally and in the United States, and tries to leave as few fingerprints as possible. Which means that even when STBF achieves major breakthroughs, it doesn't claim credit or publicly share what it's learning.

A case in point: STBF grants for reproductive health in Colorado are proving to be some of the most successful efforts ever in the U.S. at reducing unplanned pregnancies.

In Colorado, STBF's giving took the form of a program that provided long-acting birth control to women at no cost. For the past six years in Colorado, eligible women have been able to get free intrauterine devices and implants that prevent pregnancy for years.

The program made remarkable changes for teenagers and young adult women across the state. The birthrate among teens in Colorado plummeted by 40 percent from 2009 to 2013, and the rate of abortions fell by even more, with a 42 percent drop-off, according to the Colorado Department of Public Health and Environment. Similar declines in unplanned pregnancies were realized for unmarried women under 25 who have not finished high school, another group that's highly vulnerable to poverty.

STBF's work in Colorado has made a dramatic difference: 20 percent of Colorado women between 18 and 44 now use long-acting contraceptive devices, in contrast to only 7 percent nationwide.

The success of this program has heads turning to find out how the program can be replicated. It has also spotlighted the complexities of the Affordable Care Act, which is working to make contraception free and more widely available, but has yet to pass a number of hurdles, both among employers and with healthcare plans that may choose to exclude birth control.

Meanwhile, the political hurdles to publicly funding contraception are well known. Colorado's elected officials, for example, have refused to budget money for the program backed by STBF—despite evidence that "every dollar spent on the long-acting birth control initiative saved $5.85 for the state’s Medicaid program, which covers more than three-quarters of teenage pregnancies and births."

In short, when it comes to reducing unplanned pregnancies, philanthropy has a large role to play—both in advocating for policy changes and directly paying for solutions that work. Now we just need more big funders to get behind this push, joining not just STBF, but also a handful of other stars in this space, most notably the Hewlett and Packard foundations. Anti-poverty funders in particular might consider giving greater priority to reducing unplanned pregnancies, given all that we now know. This is a major leverage point in a war on poverty that, let's face it, hasn't gone so well.